[core] reinstate the call to DeletePartitions()

* But use IOCTL_DISK_CREATE_DISK and not IOCTL_DISK_DELETE_DRIVE_LAYOUT
* Also improve messaging and increase detection timeout to 15 seconds
[syslinux] fix crash when installing syslinux on large drives

* The check for minfatsize seems erroneous, and libfat_open()
  errors weren't checked in syslinux.c anyway
* Closes #156